The present invention relates to yucca6-1D, the peptide/protein YUCCA6 and methods for increasing plant growth, development and differentiation by introducing YUCCA6 into a plant. All publications cited in this application are herein incorporated by reference.
Auxin is an essential plant hormone that influences many aspects of plant growth and development including cell division and elongation, differentiation, tropisms, apical dominance, senescence, abscission and flowering (Hooley, Plant Cell (1998) 10:1581-4). Not only is auxin a plant growth regulator, it is also likely to be a morphogen (Sabatini et al., Cell (1999) 99:463-472). Although auxin has been studied for more than 100 years, its biosynthesis, transport, and signaling pathways remain elusive. In order to understand the biological functions of auxin, it is necessary to elucidate how auxin is synthesized, transported, and used as a signaling agent.
Indole-3-acetic acid (IAA), the first auxin to be chemically identified, appears to be the major endogenous auxin (Davies, The Plant Hormones: Their Nature, Occurrence, and Functions (1995) Kluwer Academic Publishers, 1-12). Based on its structural similarities, tryptophan has been proposed as the auxin biosynthesis precursor (Bartel, Ann Rev Plant Physiol (1997) 48:51-66). Many pathways have been proposed for converting tryptophan to IAA, but at present, none has been proven. Tryptophan can be converted to indole-pyruvate by transferring the amino group. Indole-pyruvate can be further converted to indole-acetaldehyde, which can be oxidized to IAA. Tryptamine, a decarboxylated product of tryptophan, has been proposed as an auxin biosynthesis intermediate.
Auxins are crucial for plant viability and development. Numerous physiological studies indicate that the major naturally occurring auxin, Indole-3-acetic acid (IAA) functions in a plethora of important aspects of plant development and growth, including apical dominance, tropic responses to light and gravity, root and shoot architecture, vascular differentiation, embryo patterning and shoot elongation (Davies, 2004). Changes in endogenous auxin levels induce genes such as SMALL AUXIN-UP RNAs (SAURs), GH3-related transcripts and AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) family members via the TIR1/AFB receptor mechanism (Dharmasiri et al., 2005; Kepinski and Leyser, 2005). The movement of auxin throughout the plant, especially by polar transport mechanisms, has been the interest of classical and current studies aimed at understanding the function of this important hormone. The quantitative temporal and spatial distributions of IAA in a plant are crucial to accomplish proper growth and development (Swarup et al., 2003; Muday et al., 2001; Blakeslee et al., 2005). Although IAA pools in a plant could be maintained at appropriate levels via auxin biosynthesis, conjugation, degradation, and transport mechanisms, de novo biosynthesis is the primal step to achieve the crucial level of auxin. However the understanding of auxin biosynthesis is still incomplete.
Analytical and feeding studies have described in detail where IAA and related compounds accumulate (Lijung et al., 2001, 2005), but application of these techniques to screens of loss-of-function mutants have not yielded enough information to fully characterize overlapping IAA biosynthetic pathways. Other efforts to dissect these pathways in Arabidopsis (Arabidopsis thaliana) have focused on isolation of mutants that are resistant to exogenously applied auxins. This approach has been highly successful for the identification of auxin receptors and elucidation of auxin signaling pathways (Estelle and Somerville, 1987; Hellman et al., 2003; Yang et al., 2004), but has contributed less to elucidating IAA biosynthetic pathways.
A more productive avenue of research has been the identification and characterization of loss-of-function mutants exhibiting altered growth phenotypes. Auxin overproduction mutants such as supperroot1 (sur1) and supperroot2 (sur2) have been identified and characterized from Arabidopsis. These mutants were isolated from loss-of-function screening, because the loss of their functions attenuates depletion of auxin levels (Bak et al., 1998, 2001; Mikkelsen et al., 2000), indicating that gene products involved directly in auxin biosynthesis may be redundant.
Recently, application of a gain-of-function approach, activation tagging, in Arabidopsis has led to breakthroughs in the study of IAA biosynthesis. In independent efforts, activation tagging revealed five loci in Arabidopsis that encode proteins affecting auxin biosynthesis (Zhao et al., 2001; Marsch-Martinez et al., 2002; Woodward et al., 2005). These loci have been categorized into the YUCCA family of flavin monooxygenase (FMO)-like proteins. This family includes 11 members in the Arabidopsis genome (Zhao et al., 2001; Cheng et al., 2006). Activation-tagged yucca mutants exhibit signature phenotypes found in auxin overproduction mutants such as sur1 and sur2, and transgenic plants that overexpress the Agrobacterium tumefaciens phytohormone-biosynthetic gene iaaM (Zhao et al., 2001; Marsch-Martinez et al., 2002; Woodward et al., 2005). Double, triple, and quadruple mutants of YUCCA family members display dramatic developmental defects that are rescued by the bacterial auxin biosynthesis gene iaaM. This reverse genetic study along with previous work by Zhao et al. (2001) has revealed not only the functional redundancy but also some functional and physiological specificities among YUCCA members. Furthermore, the involvement of YUCCA in auxin biosynthesis has also been shown in rice (Oryza sativa) and petunia (Petunia hybrida; Tobeña-Santamaria et al., 2002; Yamamoto et al., 2007). Although it is clear that the YUCCA genes play critical roles in maintaining auxin levels in plants, the cellular and biochemical characteristics and specific functions of each family member have not been fully elucidated. In addition, although YUCCA1 recombinant protein was reported to have enzymatic activity, no reports of catalytic functions of other YUCCA proteins have appeared.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.